1. Field of the Invention
The present invention relates to an optical communication system, and in particular to a wideband amplifier with at least one erbium-doped fiber.
2. Description of the Related Art
Recently, exponential growth in data usage, transfers, etc., has forced wavelength division multiplexing (WDM) optical communication systems to expand their transmission bandwidth. For this reason, wideband communication systems that simultaneously use a C-band between 1530 nm to 1560 nm (conventional band), an L-band between 1570 nm to 1600 nm (long band) and an S-band between 1450 nm to 1500 nm have been studied. Fiber amplifiers that function to amplify optical signals in optical communication systems that included erbium-doped fiber amplifiers (EDFAs) have been widely used. Such EDFAs have a bandwidth limited to about 30 nm with respect to both the C-band and L-band. The S-band has also been used as an EDFA amplifiable band, but the EDFA had difficulty in expanding the S-band. Therefore, other studies have been made regarding a thulium-doped fiber amplifier (TDFA), in which the element, thulium, is used as a new amplifiable medium. However, such TDFAs have a problem, wherein an available pumping light source generally has a wavelength of 1.05/1.56 μm or 1.4/1.56 μm, but a high power laser diode for generating such an wavelength of light is not commercialized yet.
FIG. 1 shows a conventional thulium-doped fiber amplifier (TDFA). The TDFA includes a pump module 110 having a distributed feedback (DFB) laser diode 112 and an erbium-doped fiber amplifier (EDFA) 114, a first and second wavelength selective couplers (WSCs) 120 and 150, a first and second isolators 130 and 170, a pumping light source 140 and a thulium-doped fiber (TDF) 160.
In operation, DFB laser diode 112 outputs first pumping light at a wavelength of 1.56 μm. Because the first pumping light has an output lower than a desired output, the output of the first pumping light must be increased. Therefore, EDFA 114 amplifies and outputs the first pumping light. EDFA 114 includes an erbium-doped fiber, a laser diode for outputting pumping light at a wavelength of 0.98 μm to pump the erbium-doped fiber, and a wavelength selective coupler for combining the first pumping light with the power of the erbium-doped fiber. First WSC 120 combines input optical signals belonging to the S-band with the first pumping light and outputs the combined resultants. First isolator 130 is interposed between first WSC 120 and TDF 160 and isolates backward light traveling in a direction opposite to the optical signals. Pumping light source 140 outputs second pumping light at a wavelength of 0.98 μm to pump TDF 160. Second WSC 150 combines the inputted optical signals and first pumping light with the second pumping light and outputs the combined resultants. TDF 160 is pumped by the first and second pumping light, and amplifies and outputs the optical signals. Second isolator 170 is disposed in the rear of TDF 160 and isolates backward light traveling in a direction opposite to the optical signals.
As mentioned above, the conventional TDFA makes use of pumping light at a wavelength of 0.98/1.56 μm, and uses a commercialized laser diode as the pumping light source. In this case, a commercialized high power laser diode may be used as the pumping light source generating pumping light at a wavelength of 0.98 μm, but a high power laser diode generating pumping light at a wavelength of 1.56 μm is not commercially available at this time. For this reason, pumping light outputted from a typical low power DFB laser diode is amplified by the EDFA, and such amplified pumping light is often used. For this reason, both a DFB laser diode for 1.56 μm and an EDFA are needed separately. This leads to a high price load, a large volume, and difficult system integration.